The invention is in the field of flow meters, more particularly mass flow meters of the Coriolis type.
Coriolis-type meters are based on the physical principle of conservation of angular momentum as it applies to the Coriolis acceleration of a fluid flowing through a conduit. For example, as illustrated in Sipin U.S. Pat. No. 3,485,098, when fluid flows through a bent tube which is mechanically oscillated about an axis passing through the inlet and outlet ends of the tube, Coriolis forces are generated causing the tube to twist about a response axis. Mass flow rate can be deduced from measuring this twist.
In Coriolis-type meters an electromagnetic oscillator can drive the tube about its oscillation axis at the system's resonant frequency, thereby producing a Coriolis acceleration and a resulting force. The force acts perpendicular to the flow path and in alternate opposite directions as between the two legs of the tube, superimposing an oscillating moment about the response axis on the motion about the oscillation axis. The resulting moment, acting about the response axis and in a plane perpendicular to the driving moment, produces twisting at a deflection angle which is approximately proportional to the mass flow rate for a constant angular velocity. Control over variations in angular velocity can be attempted by a detection scheme which senses the deflection angle near the central position of the tube excursions, i.e. where the angular acceleration of the tube is near zero, at the point of constant angular velocity. The tube can be oscillated relative to a leaf spring of a similar mass, to make use of a convenient resonant frequency, or relative to a parallel tube carrying the same or shared fluid flow.
Certain tube shapes have been proposed in the past. For example said Sipin U.S. Pat. No. 3,485,098 shows a tube having a U-shaped operative portion while Sipin U.S. Pat. No. 4,559,833 illustrates an S-shaped conduit. Smith U.S. Pat. Re. 31,450 illustrates a U-shaped tube supported at its inlet and outlet ends and oscillated relative to a leaf spring about an axis perpendicular the legs at the support points. In addition, said patent Re. 31,450 and Cox et al. U.S. Pat. No. 4,127,028 illustrate a tube shape which also is U-shaped but its legs are closer to each other at their inlet and outlet ends than at the curve of the U. Still in addition, said Cox et al. patent illustrates in FIGS. 4 and 5 a generally 0-shaped tube having an inlet and an outlet which in FIG. 4 are approximately radial, and in FIG. 5 are approximately tangential (but the tube is not in a single plane). Two U-shaped, or generally U-shaped, tubes carrying the same or shared flow are illustrated in said Cox et al. patent and in Smith et al. U.S. Pat. No. 4,491,025.
Various techniques have been proposed for deducing mass flow rate from measurements of the effect of Coriolis forces on the tube or tubes. For example, said Sipin U.S. Pat. No. 3,485,098 discusses using strain gauges or magnetic vibration velocity sensors to derive electrical signals related to the motion of the vibrated tube, noting in connection with velocity sensors that their differential output is proportional to mass flow rate. Said Smith U.S. Pat. Re. 31,450 states that while there may be worthwhile information obtained by measurements as in said Sipin U.S. Pat. No. 3,485,098, velocity sensors require measurement of a minute differential velocity superimposed on the very large pipe oscillation velocity. Patent Re. 31,450 therefor forsakes the use of velocity sensors in favor of optical sensors (photo-interrupters) which have a flag (an opaque plate) affixed to the oscillated tube and a photocell and a light source affixed to a stationary frame such that the sensor would detect the passage of a tube leg through a plane fixed in space but would not detect any other aspect of the tube movement. The time lag between the respective passage of each leg of the tube through a respective plane fixed in space, is proposed as a measure of mass flow rate. A later Smith et al. U.S. Pat. No. 4,442,338 proposes the use of velocity sensors (despite the comments on such sensors in Re. 31,450) or strain gauges, or acceleration sensors. It proposes squaring the sinusoidal outputs of the velocity sensors to obtain the exact same square waves as in said earlier patent Re. 31,450, and deducing mass flow rate in the same manner.
Additional examples of material concerning mass flow meters can be found in Young, A. M., "Coriolis-Based Mass Flow Measurement," Sensors, Dec. 1985, Vol. 2 No. 12, pp. 6-10; "Mass Flow Meters," Measurements & Control, Sept. 1985, pp. 295-302; Spitzer, D. W., "Mass Flowmeters," Industrial Flow Measurement, IRP Student Text, Section 12, pp. 133-141; Hickl, E. L. et al., "Mass Flow Measurement In The 80's," pp. 49-52; "Mass Flow Meters," Section 13, pp. 141-157; "Mass Flowmeter Accurate To.+-.0.15%," Chemical Engineering, Dec. 10, 1984; "Flowmeter Installs Directly In-Line With Process Piping," Chemical Processing, Mid-Nov. 1984, p. 82, DeCarlo, J. P., "Mass-Flow Measurement," Fundamentals Of Flow Measurement, 1984, Unit 11, pp. 203-220; Plache, K. O., "Coriolis/Gyroscopic Flow Meter," Mechanical Engineering, March 1979, pp. 36-41.
It is believed that a substantial need still remains to suppress undesirable characteristics of the known Coriolis-type mass flow rate meters and enhance desirable characteristics, and this invention is directed to meeting that need.
In one exemplary embodiment, a mass flow rate meter in accordance with the invention uses a pair of conduits which have a complex shape referred to in this specification as "double-pigtail", and deduces mass flow rate from changes in the phase difference of signals derived from velocity sensors responsive to relative motion between the sides of the two moving conduits.
A meter embodying this example of the invention brings about significant and surprising advantages in accuracy, ease of manufacture and use, and other desirable characteristics as compared with known prior proposals such as the use of U-shaped tube and time lag measurements of the passage of the sides of a U-shaped tube through planes fixed in space, as proposed in said U.S. Pat. Re. 31,450. For example, other things being equal, this example of the invention has a lesser dimension in the direction transverse to the general direction of the incoming and outgoing flow. This can be important in practice because a common application of Coriolis-type meters is in a continuous process, and plant pipe systems tend to be more cramped by parallel pipes in the general direction of process flow, but it is usually convenient to replace a section of pipe by a meter having the dimensions and configuration of a meter shaped as described in this application. Another significant advantage is in the type of loading at the points where the inlet and outlet ends of each meter conduit are affixed to supports. Each meter conduit typically is welded to one or more fixed supports at its inlet and outlet ends. These welds can be a weak point. In the driving mode a U-shaped conduit stresses the joint weld in bending stress while a meter shaped as described in this application stresses this joint in torsion, which is less likely to cause joint failure. In the response mode, a U-shaped meter stresses the weld joint in torsion while a meter shaped as described in this application stresses that joint in bending. However, in the response mode the bending stress on the weld joints of a meter shaped as described in this application is lower because while in the U-shaped conduit the twisting of the curve of the U is transmitted directly to the weld joint by the straight or substantially straight legs, in a meter shaped as described in this application this twisting is transmitted through continuously and smoothly curved portions which themselves twist and thus substantially reduce the bending load at the weld joints. In addition, the movement of the conduits in the response mode in this system is small as compared to the movement in the drive mode, resulting in a much reduced bending load at the weld joints as compared with a U-shaped meter. Still in addition, the response/drive natural frequency of vibration ratio in a meter embodying an example of the invention herein can be about 1:1.6, which is considerably lower than in a U-shaped meter. This lower response/drive natural frequency ratio brings about other advantages: (i) there is more phase shift for a given mass flow rate and (ii) twisting is easier about the torsional response axis. Still in addition, in a meter using the shape described herein the conduit is continuously and smoothly curved, to reduce flow resistance and disturbance. For example, the bend radius of a conduit in a meter embodying an example of the invention preferably is about, and no less than, 2 to 3 times the outside diameter of the conduit, thus assuring low resistance to flow, and low pressure drop across the meter. The use of such a bend radius in this system further helps reduce pressure drop as compared with a U-shaped conduit, in which the bend radius is considerably larger, and hence more tubing is used to make the bend. In addition, it has been found that the shape of the conduit in a double-pigtail-shaped meter inherently increases sensitivity as compared with known U-shape designs; this means that for a given level of sensitivity a double-pigtail-shape meter can use larger diameter conduits, and thus can have less flow resistance and less pressure drop. The advantages of the double-pigtail-shaped meter as opposed to prior proposals such as for an S-shaped meter include, among other things, (i) the fact that the inlet and outlet sections are coaxial, which is desirable when the meter is used in a facility where it is spliced in a straight pipe, and (ii) that a double-pigtail-shaped meter can more effectively eliminate any adverse effect of gas bubbles which may form in a liquid flow, and of liquid (such as a condensate) which may form in a gas flow. The advantages over prior proposals such as the use of generally O-shaped tubes include the fact that if the inlet and outlet are approximately radial, as in FIG. 4 of said Cox et al. U.S. Pat. No. 4,127,028, the joint weld and flow resistance and disturbance problems are similar to those discussed in connection with U-shaped tubes, and if the inlet and outlet are as in FIG. 5 of the same Cox et al. patent, there is greater asymmetry with respect to a plane normal to the oscillation axis and bisecting the conduit, which could introduce extraneous vibrational modes. Other significant and surprising advantages will become apparent from the detailed disclosure below of an exemplary double-pigtail-shaped meter.
In an exemplary embodiment of the invention, two generally double-pigtail-shaped conduits share the flow, typically equally. Usually, they are positioned with the inlet and outlets substantially horizontal. Each conduit has a large, generally C-shaped central portion. When at rest, these C-shaped portions conform to two respective substantially parallel vertical planes, and are aligned with each other in all views. The ends of the C-shaped portion merge into respective J-shaped, offset portions which are between the two C-shaped portions in plan and in front and back elevation, and partly within the C-shape in side elevation, with the free ends of the offset portions extending laterally out of the respective C-shaped portions in side elevation. These free ends of the two conduits are braced to each other just outside the C-shaped portions in side elevation. The J-shaped, offset portion at the inlet end of the C-shaped portion comprises a substantially straight inlet merging into a first flow reversing bend. In the flow direction this is followed by a second flow reversing bend, which is a part of the C-shaped portion, by a substantially straight middle part of the C-shaped portion, by a third flow reversing bend which also is a part of the C-shaped portion, and by the offset portion at the outlet side of the C-shaped portion, which comprises a fourth flow reversing bend and a substantially straight outlet. All flow reversing bends in a conduit curve in the same sense (i.e. all curve clockwise or all curve counterclockwise, depending on the side elevation chosen for the view), and the flow in the two conduits is in the same direction, and typically is shared substantially equally.
A driver alternately pushes apart and pulls together the centers (in side elevation) of the C-shaped portions of the conduits at a driving frequency which corresponds to the natural vibration frequency of the system, to thereby oscillate each conduit about a respective oscillation axis which is substantially concentric with the inlet and outlet of that conduit. If there is no fluid flow through the conduits, their substantially straight middle parts tend to remain substantially parallel despite their driving mode oscillation. If there is fluid flow through them, the resulting Coriolis forces twist each conduit about a respective response axis which is perpendicular to its oscillation axis and its substantially straight middle part, superimposing this twisting motion on the driving mode oscillation. The conduits twist about their respective response axes out of phase with each other, i.e. while their second bends (e.g. the left sides of the C-shaped portions) move toward each other their third bends (the right sides of the C-shaped portions) move away from each other, and while their second bends move away from each other their third bends move toward each other. Sensors at the second and third bends of the conduits produce sensor signals related to the relative movement of the second and third bends of the conduits toward and away from each other. The parameter of interest is the change in phase difference between the sensor signal for the second bends and the sensor signal for the third bends. Mass flow rate is deduced from this change in phase difference.